1. Field of the Invention
The invention is related to a semiconductor device and a fabricating method thereof, and in particular to a thin film transistor and a fabricating method thereof.
2. Description of Related Art
In recent years, as optoelectronic technology and semiconductor fabrication technology increasingly mature, development of flat panel displays has boomed. Since liquid crystal displays have advantages such as low operating voltage, no radioactive emissions, light weight, and small volume, liquid crystal displays have gradually replaced conventional cathode ray tube displays to become the mainstream product. Generally, liquid crystal displays may be classified into amorphous silicon thin film transistor liquid crystal displays (a-Si TFT-LCDs) and low temperature poly-silicon thin film transistor liquid crystal displays (LTPS TFT-LCDs).
FIG. 1A is a schematic cross-sectional view of a conventional thin film transistor. Please refer to FIG. 1A., a thin film transistor 100 is disposed on a substrate 101 and includes a gate 110, a patterned amorphous silicon layer 120 (which is a channel layer), a source 130, and a drain 132, wherein each of the source 130 and the drain 132 is connected to the channel layer 120 through an ohmic contact layer 140. When a high voltage is applied to the gate 100, the thin film transistor 100 is turned on, so that the channel layer 120 is in a conductive state, thereby connecting the source 130 and the drain 132. However, when a high voltage is not applied to the gate 110, the thin film transistor 100 is turned off, a leakage current is often generated by the channel layer 120 as the channel layer is irradiated by external light, thereby affecting reliability of the thin film transistor 100. In order to reduce leakage currents, conventional art provides an improvement method by performing a plasma treatment during the process of fabricating the thin film transistor 100. FIG. 1B is a schematic view of a conventional plasma treatment. Please refer to FIG. 1B, the plasma treatment is performed after the ohmic contact layer 140 and a photoresist layer 150 are formed and before a second metallic layer that is used to form the source 130 and the drain 132 is deposited. A plasma 160 used in the plasma treatment process may be an oxygen plasma (O2-plasma) or an argon plasma (Ar-plasma). After the plasma treatment is performed, a layer of silicon oxide (SiOx) thin film is formed on sidewalls 121 and 123 of the channel layer 120, so as to reduce the leakage current.
FIG. 1C is a schematic cross-sectional view of a conventional thin film transistor. Please refer to both FIGS. 1A and 1C, the difference between FIGS. 1A and 1C lies in that a source 170, a drain 172, and an ohmic contact layer 180 of the thin film transistor 102 are fabricated by using the same photolithography and etch process (PEP), so that the source 170 and the drain 172 have substantial the same pattern as the ohmic contact layer 180. However, in the thin film transistor shown in FIG. 1C, the drawbacks resulted from leakage currents still cannot be effectively solved.